A regular nut runner tightens one bolt with one drive source. However, tightening efficiency would be improved if it were possible to tighten multiple bolts at the same time with one drive source. In view of this, a two-nut runner is proposed in Japanese Utility Model Laid-Open Publication No. 61-169534. This two-nut runner is described hereinbelow with reference to FIG. 10 hereof.
The two-nut runner 100 shown in FIG. 10 is composed of a motor 101 as a drive source, a drive gear 102 mounted at the end of the output shaft of the motor 101, a first driven gear 103 fitted with the drive gear 102, a first nut runner 104 driven by the first driven gear 103, a first socket 105 mounted at the lower end of the first nut runner 104 to face towards the surface of the diagram, a second driven gear 106 fitted with the first driven gear 103, a second nut runner 107 that is driven by the second driven gear 106 and that is provided in parallel with the first nut runner 104, a second socket 108 mounted at the lower end of the second nut runner 107 to face towards the surface of the diagram, and a frame 109 for supporting the first nut runner 104 and the second nut runner 107.
The operation and problems of the two-nut runner 100 are described with reference to FIG. 11.
Two bolts 111, 112 can be tightened at the same time by threading a first bolt 111 over a boss 113 with the first socket 105, and threading a second bolt 112 over a boss 114 with the second socket 108, as shown in FIG. 11.
However, differences sometimes arise between the time that the first bolt 111 begins to be threaded into a female screw 115 and the time that the second bolt 112 begins to be threaded into a female screw 116. Tightening of the two bolts 111, 112 then does not finish at the same time. In this example, it is assumed that the first bolt 111 is completely tightened before the second bolt 112.
In FIG. 10, the first socket 105 stops when tightening is complete. The first driven gear 103, the second driven gear 106, and the second socket 108 then also stop because these components are all mechanically linked together.
As a result, the second bolt 112 is not yet finished being tightened. The second bolt 112 must then continue to be tightened. The same applies in cases in which the second bolt 112 finishes being tightened before the first bolt 111, and the first bolt 111 must continue to be tightened. Thus, additional tightening is required, and operating efficiency is reduced.
In view of this, there is a demand for a bolt-tightening tool that can tighten two bolts with equal torque in a single operation.